A wide range of electrical requirements must be met by antennas in wireless devices. These requirements include TRP (total radiated power), TIS (total isotropic sensitivity), efficiency, and SAR (specific absorption rate). The TRP is a measure of the radiation efficiency of an antenna; the SAR is a measure of the density of the near-field field strength as measured in human tissue adjacent to the antenna enabled device. An improvement in SAR, which is a reduction in SAR value, typically coincides with reduced radiating efficiency. It is highly desirable to develop methods to reduce SAR without impacting antenna radiating efficiency.
An antenna positioned on a small to moderate sized wireless device such as a cell phone, laptop, USB dongle, or data card excites the circuit board and other components of the wireless device. The near field electromagnetic field distribution and far field radiation pattern characteristics are affected by the characteristics of the wireless device.
In order to achieve good efficiency and SAR from an internal antenna, techniques need to be developed to reduce the amount of near field coupling of the antenna to the user while maintaining good antenna efficiency. This can be achieved by modifying the near field of the combination of the antenna and wireless device by spreading the regions of peak electric and magnetic field strength over a larger volume. This approach reduces the electromagnetic field strength per unit volume in the near field of the wireless device. If the near field distribution can be spread over a larger volume without reducing antenna efficiency then the desired outcome is achieved.